The present invention relates to improvements in large aperture-ratio standard lens systems, particularly lens systems of the deformed Gauss type for use with single lens reflux cameras, and composed of seven lenses in six components.
Generally, a Gauss type or deformed Gauss type lens is used as the standard large aperture-ratio lens in a single lens reflex camera because such lenses permit a large aperture-ratio with a relatively long back focus.
U.S. Pat. Nos. 4,099,843 and 4,110,007, U.S. patent application Ser. No. 891,024 now U.S. Pat. No. 4,247,171, and Japanese patent applications Laid-Open Nos. Sho 53-69031 and Sho 53-117420, disclose various attempts at correcting aberrations in such lenses.
However, even in a Gauss type lens consisting of seven lenses in six components and having a superior picture quality, aperture-ratios as large as F=1:1.2 result in increases in various aberrations, especially off-axial halo and curvature of field. This lowers the contrast of the picture or the resolving power of the lens.
An object of the present invention is to provide a standard angle large aperture lens for single lens reflux cameras.
Another object of the present invention is to correct various aberrations, particularly off-axial halo and curvature of field.
According to an aspect of the invention, the lens is composed of seven lenses in six components. The first component consists of a positive meniscus lens having a convex surface toward the object, the second component a positive meniscus lens having a convex surface toward the object, the third component a negative meniscus lens having a convex surface toward the object, the fourth component a negative meniscus lens consisting of a negative and a positive lens cemented to each other and having a convex surface toward the image as a whole, the fifth component a positive meniscus lens having a convex surface toward the image and the sixth component a bi-convex lens, with the following conditions:
(1) 1.75&lt;(N1+N2+N5+N6+N7)/5 PA0 (2) (.nu.1+.nu.2+.nu.5+.nu.6+.nu.7)/5&gt;45. PA0 (3) 1.85&lt;n6 PA0 (4) .nu.4&lt;26. PA0 (5) n3&lt;1.67 ##EQU2## (7) 0.30f&lt;D6&lt;0.35f (8) 0.4/f&lt;.vertline..phi.4+.phi.5.vertline.&lt;0.6/f, .phi.4+.phi.5&lt;0 PA0 (9) 0.68f&lt;.vertline.R11.vertline.&lt;0.73f, R11&lt;0 .